How well does an SSD Cache work?

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How well does an SSD Cache work?

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I’m thinking of buying a couple of NVMe disks to use for cache in my DS1019+, and wondered what other people’s experiences are with them?

I remember reading in the past that cache disks on DSM6 don’t (or didn’t) work particularly well, but should be improved in DSM7. But it was quite a while ago when I read that, and wondered if cache disks in DSM6 work well lately? My read speeds are good, but write speeds aren’t great so wondered if adding cache disks could improve performance.
 

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If you're bothered by the write (or read speeds even) a Read/Write SSD cache is the way to go. I suggest when buying get 3x for 2 slots as they install with a recommended RAID1 config so if one fails you swap in the spare 3rd you have stored in your drawer with Slot x. Remember that the SSD cache stores your most common accessed data during the last week, and redudancy is important even here to avoid data-loss.

Enjoy

//Ex
-- post merged: --

May I ask what kind of RAID array config you're using @jono ?

Sincerely,
Exsosus
Synology DSM Advisor
 
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If you're bothered by the write (or read speeds even) a Read/Write SSD cache is the way to go. I suggest when buying get 3x for 2 slots as they install with a recommended RAID1 config so if one fails you swap in the spare 3rd you have stored in your drawer with Slot x. Remember that the SSD cache stores your most common accessed data during the last week, and redudancy is important even here to avoid data-loss.

Enjoy

//Ex
-- post merged: --

May I ask what kind of RAID array config you're using @jono ?

Sincerely,
Exsosus
Synology DSM Advisor
Thanks, sounds good. It’s set up with SHR.
 

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It
Thanks, sounds good. It’s set up with SHR.
depends on the data type and amount of usage if you will or will not see a perf boost. If you are running some apps/services that use database type data, or with a large number of repeating small file access, then you will see the benefit. If you are just working with large files (several GB in size or more) then you will not see the benefit at all.

The OS will feel a lot snappier if nothing else that's a fact, but like I said it's all up to the way what role that NAS is designed to do.

If you are running already an SSD array inside your NAS then you will have no benefit from the cache (nothing noticeable) but in any other case, it is safe to say you will see more or less speed boosts.
 
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Thanks Rusty. It has regular HDDs, not SSDs, and I’m working mainly with lots of small files, with not so many large files.

Running the adviser it recommends a cache of 700GB. Does that (for example) mean a single 1TB SSD for read and write, or would it be better to get 2 SDDs, 1 for read and 1 for write?
 

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Thanks Rusty. It has regular HDDs, not SSDs, and I’m working mainly with lots of small files, with not so many large files.
Yep in that case you have a reason to expect speed boosts
 
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small files accessed a lot should see a benefit.

For me, the primary use of my NAS is a plex server with multi gigabyte files - i would see zero benefit.
some have suggested that NVMe cache might improve the speed that the coverart loads in the plex clients, but any example i've seen (on youtube etc) the improvement has been so slight that it's not noticeable.
 

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small files accessed a lot should see a benefit.

For me, the primary use of my NAS is a plex server with multi gigabyte files - i would see zero benefit.
some have suggested that NVMe cache might improve the speed that the coverart loads in the plex clients, but any example i've seen (on youtube etc) the improvement has been so slight that it's not noticeable.
I would agree with this. I did notice the difference tbh, but not something drastic. The main boost that I have noticed when I moved the Plex server-side to an all-flash array and on top of that a new rewritten Samsung TV Plex client. Now the UI is super responsive is fast.
 
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Thanks guys!

If the adviser it recommends a cache of 700GB would it be better to get a single 1TB disk, 2 500GB disks (1 for read, 1 for write), or does it mean 700GB for read and 700GB for write? (So 2 x 1TB)
 

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It means 700GB of data in total. So you are looking at 2x1TB. Also, one drive will not be for read and the other for write. The cache can be either in raid0 (2x1TB for read cache) or raid1 (2x1TB for r/w cache).
 
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Ah, great. Thanks for the clarification (y)
 
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something tells me i won't get investing in a cache any time soon :ROFLMAO:

no idea why there is a jump of 4TB from yesterday to today.

Screenshot 2020-11-26 143750.jpg
 

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no idea why there is a jump of 4TB from yesterday to today.

Heh....
Maybe a full backp ran?
Or a scheduled anti-virus fullscan?
 
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nope, none of those.

I'm assuming the "total file size" is the total amount accessed that day, as i have way more than 4.66TB total used on the volume.
 

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I think it adds up the last seven days that is shown in 'Total File Size'. Give it a week and don't shift TB of data.
 
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i never "shift" much data about at all.
it's primarily a plex server, so new movies/tv shows get downloaded all the time, but they get downloaded to the NAS and then moved from the download folder to the media folder.

certainly hasn't been TBs worth a data moved about or added in the past 7 days (or even 30 days).
 

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The screenshot has 4.66 TB for today. That’s why it recommends 4.8 TB for SSD cache.
 
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small files accessed a lot should see a benefit.

For me, the primary use of my NAS is a plex server with multi gigabyte files - i would see zero benefit.
some have suggested that NVMe cache might improve the speed that the coverart loads in the plex clients, but any example i've seen (on youtube etc) the improvement has been so slight that it's not noticeable.

I'm not an expert, but I'm not sure this is always true.
It depends on the degree of fragmentation and how full the pool is.

The next is about ZFS, but probably is the same in BTRFS and every COW filesystem.
Of note snapshots contribute further to fragmentation.

A 10%-full pool is going to fly for writes. By the time you get up to 50%, the steady state performance is already pretty bad. Not everyone is going to get there... if you have a lot of content that is never rewritten, your fragmentation rates may be much better because you haven't rewritten as much stuff.

Particularly noteworthy: The pool at 10% full is around 6x faster than the pool at 50%.

But what about reads? We've spent all this time talking about writes and free space. ZFS rewards you with better write speeds if you give it gobs of free space. Reads still suffer from fragmentation and seeks!

from this very interesting resource: The path to success for block storage
 
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Thanks Rusty. It has regular HDDs, not SSDs, and I’m working mainly with lots of small files, with not so many large files.

Running the adviser it recommends a cache of 700GB. Does that (for example) mean a single 1TB SSD for read and write, or would it be better to get 2 SDDs, 1 for read and 1 for write?
Hi, that means: 2 SSDs in RAID1, same size: 1TB each , and choose the mode: Read/Write Cache setup when configuring the cache.
-- post merged: --

(Buy 3, 1 on your shelf in storage) So when SSD drive X dies, you just swap it in and it will rebuild the RAID1.
 

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